問題背景
1) 限制:每個用戶在一個類目下,最多享受4單滿送優惠。
即統計享受了滿送優惠的訂單數時需要排除掉同一類目下第四單以后的訂單
如何得到每個用戶同一類目下第四單ID呢?
select?fourth(id)?from?order?group?by?user_id,?category;
可惜mysql并沒有分組函數可以方便的得到每組中第四個id。但可以有變通方案, 如有如下的表
select?*?from?t; +----+--------+----------------------------------+ |?id?|?status?|?user_id??????????????????????????| +----+--------+----------------------------------+ |??1?|?10?????|?24e568a88fae11e6bb0650b497d97cdd?| |??2?|?10?????|?24e568a88fae11e6bb0650b497d97cdd?| |??3?|?20?????|?24e568a88fae11e6bb0650b497d97cdd?| |??4?|?20?????|?24e568a88fae11e6bb0650b497d97cdd?| |??5?|?10?????|?e8669ac28fae11e6bb0650b497d97cdd?| |??6?|?20?????|?e8669ac28fae11e6bb0650b497d97cdd?| |??7?|?10?????|?e8669ac28fae11e6bb0650b497d97cdd?| |??8?|?20?????|?e8669ac28fae11e6bb0650b497d97cdd?| |??9?|?10?????|?e8669ac28fae11e6bb0650b497d97cdd?| +----+--------+----------------------------------+
如何得到每個用戶第二次成功支付(status=’10’)的訂單呢?
方法一 — 逐個查詢
select?*?from?t?where?user_id?=?'24e568a88fae11e6bb0650b497d97cdd'?and?status?=?'10'?order?by?id?limit?1,1; +----+--------+----------------------------------+ |?id?|?status?|?user_id??????????????????????????| +----+--------+----------------------------------+ |??2?|?10?????|?24e568a88fae11e6bb0650b497d97cdd?| +----+--------+----------------------------------+
方案二 — 一次性查詢
SELECT? ????* FROM ????(SELECT? ????????id, ????????????user_id, ????????????@rank:=IF(@current_user_id?=?user_id,?@rank?+?1,?1)?rank, ????????????@current_user_id:=user_id ????FROM ????????(SELECT?@current_user_id:=NULL,?@rank:=NULL)?vars,?t ????WHERE ????????t.status?=?'10' ????ORDER?BY?user_id?,?id)?a WHERE ????rank?=?2; ???? +----+----------------------------------+------+----------------------------------+ |?id?|?user_id??????????????????????????|?rank?|?@current_user_id:=user_id????????| +----+----------------------------------+------+----------------------------------+ |??2?|?24e568a88fae11e6bb0650b497d97cdd?|????2?|?24e568a88fae11e6bb0650b497d97cdd?| |??7?|?e8669ac28fae11e6bb0650b497d97cdd?|????2?|?e8669ac28fae11e6bb0650b497d97cdd?|
原理解釋
先按user_id和id順序排序, 然后從1開始分配序號,同一用戶就遞增,不同用戶則重新從1開始。
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